Low-speed flight phases are dangerous for aircraft, in particular during landing phases where the margin for manoeuvre in relation to the terrain is all the more critical the lower the visibility and the altitude. To prevent abnormal changes of altitude and of attitude of the aeroplane, detection systems with terrain databases exist. But coupled with a disabling system and in the environs of an airport, they turn out to be inoperative. They then no longer offer the pilot the possibility of rescuing the aeroplane early enough before a collision with the ground in the case of an abnormal drop.
According to the known prior art, onboard terrain monitoring systems are based on simulated probers projected in front of the aircraft and derived from the speed of the aircraft with respect to the ground and the reaction time of the pilot. Whatever the altitude of the aircraft, a computer continuously verifies the rate of penetration of this prober with a terrain database, itself onboard. As soon as a nonzero penetration rate is detected and confirmed, the computer dispatches an alert to the pilot warning him of the proximity of the terrain.
A problem arises with such terrain detection systems at the moment of the approach of an airport where the aircraft is projected to land. Indeed, the altitude decreasing, the simulated probers intercept the terrain in the environs of the runway. It therefore becomes necessary to disable these alerts so as not to disturb the pilots in the course of this critical phase.
Recourse is then had to three complementary disabling methods:
The first method relies on the definition of a spatial envelope which extends in the three dimensions. This envelope is related to the runway threshold, to the precision of navigation of the aircraft, and to the geometric characteristics of the runway considered. As long as the aircraft is situated outside this envelope, the computer is authorized to output all the specified terrain alerts. The alerts are disabled when the aircraft is situated in the envelope.
The second method is related to the consistency of heading between the aircraft and the runway considered. One then speaks of convergence towards the runway in the horizontal plane. If the heading of the aeroplane does not converge quickly enough towards the axis of the runway, the computer remains potentially alerting.
Finally, the third method is based on the consistency of the vertical speed of the aircraft with respect to its altitude (with respect to the low limit of the spatial envelope). One then speaks of convergence of the vertical speed of the aircraft with respect to the ideal vertical speed at a given altitude (with respect to the low limit of the spatial envelope). At a given altitude, an aircraft whose vertical speed was outside of the theoretical envelope would see the terrain alerts appear in the case of collision of the probers with the terrain cells of the database. In this third method, disabling is authorized for downward vertical speeds even on the low limit of the spatial envelope.
Because of the uncertainties of positioning of the aircraft and runway thresholds, current onboard terrain detection systems do not make it possible to warn the pilot of the imminence of an impact as soon as the aircraft is situated in the environs of an airport or of a runway covered by this alerts disabling system.
Indeed, the vertical part of a landing runway spatial disabling envelope covers a ground area which amply exceeds that of the runway. FIG. 1 represents a landing runway 201 and a spatial disabling envelope projected onto the ground. The runway comprises a runway threshold G. The disabling zone projected onto the ground is defined on the basis:                of a first point S, situated at a distance Δ from the runway threshold G and placed so that the segment GS is situated in the longitudinal axis of the runway,        of a second point O and of a third point E, which are situated at a distance Δ from the runway threshold G and placed on a straight line perpendicular to the longitudinal axis of the runway,        of a fourth point N placed so that the segment GN follows the longitudinal axis of the runway.        
The parameter Δ is related to the precision of navigation of the aircraft. The risk of CFIT is therefore all the larger the more significant the parameter Δ (low precision).
In practice, an aircraft may find itself in an abnormal drop situation even though it satisfies all the conditions (horizontal and vertical) of convergence and therefore of disabling of the terrain proximity alerting system. Therefore in this case the aircraft is at risk of hitting the ground before the runway threshold.